Bomb Ealorimetry: The Energy Content of Pizza Roland P. Stout,' Faith E. Nettleton, and Lynn M. Price Drake University, Des Moines, IA 50311
The familiar experiments using a bomh calorimeter in the physical chemistry laboratory typically measure heats of combustion or reaction. or strain or resonance energies hut often do not stimulate much curiosity on the part ofhealthscience maiors. The experiment described here which grew out of a linchtime co&ersation several years ago, seems consistently to pique the interest of students. Its popularity, partly due to the "real-world" aspect of using a sample so familiar to college students, induces them to explore sampling and sample preparation problems not ususally considered when using the more typical, pure samples. We have also found that this experiment often initiates a spirited discussion on how accurately the measured heat of combustion reflects the caloric food value of pizza. A representative slice of pizza is selected and frozen. The frozen sample is weighed and then ground with an equal mass of water using an ordinary blender. The resultant slurry is transferred to a tared evaporating dish, weighed, and dried a t 8O0Cto constant weight, typic&y takingabout 48 h. A portion of the dried sample is ground using a mortar and pestle with an equal mass of benzoic acid which serves as a combustion aid ensuring complete comhustion of the sample. Pellets weighing 1.0-1.2 g are pressed using a drop of water if necessary to hold each pellet together. The bomb containing a pizza pellet is assembled according t o the manufacturer's instructions. The bomb assembly is flushed twice using 15-20 atm of pure oxygen t o remove atmospheric nitrogen. The bomh is then filled with oxygen to the iecommendid nressure. and the s a m ~ l eis comhusted using standard methods. As the bomb is disassembled, it is rinsed out with distilled water, and the washings are titrated with 0.02 N base. Since atmospheric nitrogen was removed from the bomh prior t o combustion, titration of the bomb washings, assumed to contain only nitric acid, yields the nercent nitroeen in the sample and a crude estimate of the percent protein in the pizza.
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Results and Discussion The combustion of the pizza-benzoic acid samples has a typiral succrss rate greaterihan 90%. Therr is usually a small amount of a light tan residue and several hnnl nodules which are very slowly soluble in water. l'hesc nodules appear to he commsed of iron oxide from thr fuse and fused salts from the pizzH sample. The heat of comhustion per gram of the sample, AE,, is determined in the usual way. The energy content of pizza, AEp, is then given by
AE,
= [(MF,)-'A&
- (MRe)(AEe)lx
where MF, is the mass fraction of dried pizza in the sample and MRB is the mass ratio of benzoic acid to dried pizza in the sample pellet, AEBis the known heat of combustion of benzoic acid, and x is the mass ratio of the dried sample to the original (wet) pizza sample. The titration data yields the number of moles of nitric acid formed. and hence the moles of nitroeen (atoms) in the sample, from which thp percent nitrogen in the sample is determined. Multiplying the percent nitrogen by 6.25 gives the percent protein in ihe sample.:' The results riven in the tnhle wercdmincd hv the authors for a frozen, sausage pizza. The measured heat okcombustion
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438
Journal of Chemical Education
Plzza Combustion Results Measured value Determinations Llterahlre values Kaldihal
AEP(k(kcalag-')
%
2.49 f 0.03b 7 2.50
3.5 0.2b
...
Protein
4
6.5 5.7
compares quite well to the food value for a similar pizza. This is found to be typical of student results which are usually within 5%of the literature value for a similar pizza, which is felt to be a reasonable estimate for the variation from one pizza to another. The measured percent protein shows good precision but is consistently around 45% low. The source of this discrepancy is not known, but some of the nitrogen may be tied up in the fused salt comhustion residue. A Keldjhal analysis4 of the pizza provided an independent check on the comhustion method for percent protein and confirms that the combustion method is kdeed Gased low. Although it is not required in the report, many students choose to compare the combustion process to the metabolic processes involved in the digestion of the pizza5 They usually point out that metaholism is not totally efficient; that the enerm recovered by metabolism in terms of ATP and other energy carriers is less than the energy available by complete oxidation of the food. In other aspects metabolism is more efficient than complete comhustion~Amino acids, for example, can he utilized directly, representing an energy gain over the deeradation of the amino acids to release enerev. -.. and the " subsequent biosynthesis of "new" amino acid molecules. The metabolic efficiency is roughly constant in humans and is usually ignored in determining the caloric content of foods, which is tvoicallv.measured bv total comhustion.6 In contrast to metabolism, digestion is a more variable proeess, depending upon the nature of the food, the amount eaten, and physiological factors induced, for example, by stress. Digestibility coefficients have been measured for many foods and average values for carbohydrate, fat, and protein are a ~ a i l n b l eUsing .~ these data and the percentage of carbohydrate, fat, and protein in pizza,7 a digestibility coefficient of 96% can be calcu- A
' Author to whom correspondence should be sent.
A handout describing the experiment in more detail and discussing k heat sampling and sample preparation and the relationship between t of combustion and the food value of pizza is available from the authors. This conversion factor from the Keldjhal literature is for general proteins. See for example Pearson. David. "The Chemical Analysis of Foods." Chemical Publishing Co., Inc., New York, 1970, p. 9. See reference in footnote 3. For general discussions of digestion and metabolism see any general biochemistry text, for example, Stryer. L., "Biochemistry," W. H. Freeman B Co.. San Francisco, CA. 1981. Robinson. C. H., and Lawier, M. R.. "Normal and Therapeutic Nutrition," Macmillan Publishing Co., Inc., New York. 1977, pp. 118119. See reference given in table footnote c.
lated for a sausage pizza. Including this coefficient, a physiologic food value of 2.40 kcal g-' is obtained for our sausage pizza, still in good agreement with the literature value.? Conclusion
We believe this experiment to he a viable alternative to the common bomb calorimetry experiments, to he of interest to all students, and to he especially well suited for health-science students. It has proven quite popular at Drake where it has had the unanticipated benefit of linking physical chemical
measurements and metaholic biochemistry. An indication of the enthusiasm generated by this experiment is given by the ending sentence from one student's report: "What's for dessert?" Acknowledgment
We would like to thank Larry E. Brown who first proposed this experiment several years ago. Thanks is also due to the students who have tested this experiment, proposed modificationgand first asked the metaholic questions.
Volume 62
Number 5
May 1985
439
